1. Field of the Invention
This invention relates generally to amplifier design and, more particularly, to the design of AC amplifiers used in performing precision measurements.
2. Description of the Related Art
Scientists and engineers often use measurement systems to perform a variety of functions, including measurement of a physical phenomena or unit under test (UUT), test and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples.
A typical measurement system comprises a computer system, which commonly features a measurement device, or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a card or board plugged into one of the I/O slots of the computer system, or a card or board plugged into a chassis, or an external device. For example, in a common measurement system configuration, the measurement hardware is coupled to the computer system through a PCI bus, PXI (PCI extensions for Instrumentation) bus, a GPIB (General-Purpose Interface Bus), a VXI (VME extensions for Instrumentation) bus, a serial port, parallel port, or Ethernet port of the computer system. Optionally, the measurement system includes signal-conditioning devices, which receive field signals and condition the signals to be acquired.
A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a UUT.
Measurement systems, which may also be generally referred to as data acquisition systems, may include the process of converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. PC-based measurement and data acquisition (DAQ) systems and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor, among others.
Typically, in a measurement or data acquisition process, analog signals are received by a digitizer, which may reside in a DAQ device or instrumentation device. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an Analog-to-Digital Converter (ADC), and transmitted to a computer system for storage and/or analysis. Then, the computer system may generate digital signals that are provided to one or more digital to analog converters (DACs) in the DAQ device. The DACs may convert the digital signal to an output analog signal that is used, e.g., to stimulate UUT.
Multifunction DAQ devices typically include digital I/O capabilities in addition to the analog capabilities described above. Digital I/O applications may include monitoring and control applications, video testing, chip verification, and pattern recognition, among others. DAQ devices may include one or more general-purpose, bidirectional digital I/O lines to transmit and received digital signals to implement one or more digital I/O applications.
Typically, digital multimeters (DMMs) and other measurement instruments comprised in DAQ devices are required to measure AC signals with a high degree of accuracy over a wide range of signal levels and frequencies. Oftentimes the measured signal is small relative to the dynamic range that is typical of ADCs. That is, the measured signal may have a small dynamic range, for example on the order of tens of mV in some systems. In other cases the measured signal may need to be attenuated, as it may be too high for the given ADC. Therefore it is generally required to further process the measured signal in order to match the dynamic range of ADCs. To achieve this, the measurement instruments may include switchable attenuators and amplifiers to scale the measured signal to a level appropriate for the ADC or RMS-to-DC converter used in the measurement. Characteristically, these attenuators and amplifiers are expected to not present a heavy load to the signal source while preserving the frequency content of the signal. Maintaining a desired frequency response in a high-impedance (1 MΩ or higher) environment presents a sizeable challenge in DMM design. The problem may be especially acute considering the need to attenuate a measured signal before buffering, when high voltage levels are involved.
A typical front end for a DMM may include a switchable precision 1 MΩ or 10 MΩ resistive divider incorporated into the AC signal path, as shown in FIG. 1. The resistive divider comprises resistors 114 and 112, coupled to a programmable gain amplifier (PGA) 102 via high-voltage switch 106, with the output of PGA 106 coupled to RMS measurement circuit 104. Input signal 150 coming into the circuit is coupled to the resistive divider via DC coupling switch 118 having an AC coupling capacitor 116 across its terminals. In this case, the parasitic capacitances (110a and 110b) of the divider make it difficult to achieve a flat frequency response, and some adjustment is required, implemented here by way of an adjustable capacitor 108 to provide compensation for the parasitic capacitance (110a and 110b).
As shown in FIG. 2, other DMM designs may feature completely separate DC and AC signal paths, using a separate 1 MΩ resistor 216 for the AC path coupling to the input of an inverting amplifier 206 configured with feedback resistor 204. This reduces the parasitic capacitance (210), though typically not enough to obviate the need for adjustment, implemented here by way of an adjustable capacitor 208 configured to provide compensation for the parasitic capacitance (210). Furthermore, the fact that all input signals, not just the large ones, may pass through the 1 MΩ resistor leads to increased noise on the more sensitive AC voltage ranges. In addition, the selected value of resistor 204 needs to be precise, since it is configured to set the gain of the circuit shown in FIG. 2.
In both approaches (shown in FIG. 1 and FIG. 2, respectively), the interaction of high resistance values with small, sometimes unintended capacitances typically results in frequency response aberrations. Presently, a variety of techniques may be used to compensate for these aberrations. These typically include canceling zeroes with adjustable poles (and vice-versa) using electronic trim-pots and DACs, and compensating for frequency response errors using digital signal processing. However, the use of these techniques generally adds complexity and increases the cost of DMM and/or measurement products.
Another drawback of most current solutions is the presence of the requisite first-order response of the AC coupling. This response does not allow for a sufficient tradeoff between settling time and a flat response at low frequencies. Sometimes the low-frequency response is corrected based on a measurement of the input frequency, further increasing circuit complexity, and subject to errors when presented with complex input signals.
Thus, there it would be beneficial to provide a simple adjustment-free AC front-end circuit for DMMs and other precision measurement devices that accommodates both low- and high-level input signals with a flat frequency response and relatively fast settling time. Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.